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how much does molly change the B.C of a bullet

 
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  #8  
Old 08-26-2004, 04:16 PM
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Re: how much does molly change the B.C of a bullet

Hera is an excerpt from a piece written by Dan Hackett. He is a regular poster at xtremeaccuracy.com


As the only person I know of who has published articles about trajectories becoming flatter with moly-coated bullets, I think I can say I know something about the topic. First, I do not merely claim a reduction in bullet drop; I report it as a matter of objective fact. This is a real phenomenon. It is not something I thought I noticed while shooting offhand at a rock on the far side of some canyon. It is an occurrence I have repeatedly observed, measured, verified, duplicated and recorded with at least as much care as Mr. Vaughn reports using anywhere in his book, including tests I have performed with 6mm benchrest bullets similar to the ones he employed.

Second, a flattening of trajectory implies an increase in ballistic coefficient only if one assumes that moly-coated bullets fit the same ballistic model as uncoated ones. During their passage through a rifle barrel, moly bullets undergo considerably less deformation of their overall shape and of the jacket surfaces which contact the bore. There is no good reason to doubt out of hand that these changes will affect airflow over the bullet, and it is possible they could change the relationship between drop and velocity loss on which standard Ingalls-style ballistic computations depend for their accuracy, without significant alteration of the drag acting to slow the bullet. It should be remembered that a variety of different drag models exist to account for various projectile shapes, but sporting bullets are all lumped into a single model for the simple (if rather lame) reason that they donít usually fit any of the other models. We canít assume that the flight of every bullet from handgun semi-wadcutter to needle-pointed VLD can be portrayed with complete accuracy in every aspect, based on the size of a single variable such as ballistic coefficient. Nor can we assume without adequate testing that the shape- and surface differences that are readily observable between fired moly- and uncoated bullets will have no effect on the shape of the trajectory curve.
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Old 08-26-2004, 11:19 PM
JBM JBM is offline
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Re: how much does molly change the B.C of a bullet

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I don't think dimples reduce golf ball drag, but instead, they increase it.
This causes the ball to curve upward due to it's backspin, raising the trajectory, and giving more 'airtime'(ie.distance).
<HR></BLOCKQUOTE>

The dimples keep the boundary layer from separating a little longer than if they weren't there. This reduces drag (reduces pressure from front back).

The curving is due to the magnus effect. Much larger force for golf balls than bullets (very different Reynolds numbers...)

As far as the roughness and rifling goes, these are way down in the boundary layer and don't really matter. Robert McCoy wrote to precision shooting about his a long time ago after someone had written an article "theorizing" that the rifling was causing some kind of inaccuracy.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
I'm sure that moly would be applied to fighter jets & rockets if it reduced drag. <HR></BLOCKQUOTE>

Yep.
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Old 08-27-2004, 08:37 AM
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Re: how much does molly change the B.C of a bullet

Hackett's rant of a "book review" of Vaughn's book goes too far. He had a few points but was overly critical, and unobjective in his defence of Moly.

Vaughn's tests didn't assume any model, he did chronograph tests. He would thus notice improvements from all possible sources (skin friction, better maintained form etc). Henry Childs (Benchrest forum) does alot of testing with chronographs also. He hasn't seen any significant change in BC with MOLY.
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Old 08-27-2004, 08:41 AM
JBM JBM is offline
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Re: how much does molly change the B.C of a bullet

I would also be skeptical of improvements from moly coating because the muzzle velocity is lowered due to combustion of the moly coating lowering the gas temperature so how much moly is still on thet bullet is debatable. Also, the coating is way thinner than the boundary layer.
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Old 08-27-2004, 12:00 PM
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Re: how much does molly change the B.C of a bullet

I'm with Brad Millard's comment on it affecting the powder charge.If you don't coat your bullets but simply drop a small amount in your powder you get the same results.How can it reduce friction from behind he bullet? In reality it slows the burning rate of the powder slightly thus giving you a longer more even push on the bullet.Have you ever noticed you need more powder with moly than without? This is the same as switching to a slightly slower powder.If Henry Childs said it you can take it to the bank. I'm in the process of dong some chronograph testing for him as i type and he is helping me design a better 50 bmg bullet for 1000 yard targets.
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Old 08-27-2004, 04:15 PM
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Re: how much does molly change the B.C of a bullet

I used to correspond with Dan Hackett from time to time as we were both on a benchrest list. Dan is a little blunt, not real patient with fools, and some people take it personal. In my dealings with him, he was always factual and if something was an opinion rather than something he had data to back up, he always made that clear. Good enough for me.

There were a lot of BR shooters and gunsmiths on the list, not just Hackett, but Bill Gammon, Jim Borden, Mark something or other who started BlackStar barrels, couple F-class shooters from europe and australia. It was a good place to shut up and listen.

The consensus was there was something going on with moly but nobody could quite pin it down. Too many cases of someone with a rifle that agg in the .2s, so it was real repeatable, having bullets drop a third or less as much with moly coating as they did naked. It wasn't experimental error. For sure nobody can push a 6ppc fast enough to explain the trajectory change. So ... something must have been happening to change the assumptions, in other words, the drag model that predicted drop for the bullets naked didn't apply correctly with moly.

That can mean a lot of different things.

I think there's enough evidence out there that even if we can't explain the mechanism, keeping an open mind is justified.

I'm not sure whether moly is worth it for most people. I've used it off and on, especially if I get a real barn-burner cartridge shooting well with one bullet and want to extend barrel length. Anything above about 3200 fps seems to copper foul with or without moly. I've had some pretty substantial gains in barrel life with it so for that reason it seems worthwhile.
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Old 08-27-2004, 05:29 PM
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Re: how much does molly change the B.C of a bullet

This was my reference for golf ball dimples: http://www.leaderboard.com/WHYDIMPS.htm

It sure sounds like dimples create a drag differential due to their spining action to me. Like a wing.
I don't know how dimples affect an object which isn't spinning. It seems that bonneville and F1 racing engineers would have jumped all over dimmples, if drag was reduced by them.
A bullet is dimpled, sort of, with rifling. Do bullets curve alittle one way or another depending on rifling direction?
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